Saturday, August 1, 2009

SN 1054

SN 1054 (Crab Supernova) was a supernova that was widely seen on Earth in the year 1054. It was recorded by Chinese, Japanese, and Persian/Arab astronomers as being bright enough to see in daylight for 23 days and was visible in the night sky for 653 days.The progenitor star was located in the Milky Way galaxy at a distance of 6,300 light years and exploded as a core-collapse supernova.

The Crab Nebula consists of a broadly oval-shaped mass of filaments, about 6 arcminutes long and 4 arcminutes wide surrounding a diffuse blue central region. The filaments are the remnants of the progenitor star's atmosphere, and consist largely of ionised helium and hydrogen, along with carbon, oxygen, nitrogen, iron, neon and sulfur.

The Crab Nebula is currently expanding outwards at about 1,500 km/s. Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its spectroscopically determined expansion velocity, the nebula's distance can be estimated. In 1973, an analysis of many different methods used to compute the distance to the nebula reached a conclusion of about 6,300 light years. Along its longest visible dimension, it measures about 13 ± 3 light years across.

Tracing back its expansion consistently yields a date for the creation of the nebula several decades after 1054, implying that its outward velocity has accelerated since the supernova explosion. This acceleration is believed to be caused by energy from the pulsar that feeds into the nebula's magnetic field, which expands and forces the nebula's filaments outwards.

At the center of the Crab Nebula are two faint stars, one of which is the star responsible for the existence of the nebula. The region around the star was found to be a strong source of radio waves in 1949 and X-rays in 1963, and was identified as one of the brightest objects in the sky in gamma rays in 1967. Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of the first pulsars(Pulsars are sources of powerful electromagnetic radiation, emitted in short and extremely regular pulses many times a second.) to be discovered.